Flash memory is a storage medium that is becoming more and more popular. Though not yet fully embraced in traditional computing systems, Flash memory is prevalent in embedded systems, materialized as commodity appliances such as the digital camera and the MP3 player that we enjoy in our everyday lives. This paper considers an issue in file systems that use Flash memory as a storage medium and makes the following two contributions. First, we identify the cost of block cleaning as the key performance bottleneck for Flash memory analogous to the seek time in disk storage. We derive and define three performance parameters, namely, utilization, invalidity, and uniformity, from characteristics of Flash memory and present a formula for block cleaning cost based on these parameters. We show that, of these parameters, uniformity most strongly influences the cost of cleaning and that uniformity is a file system controllable parameter. This leads us to our second contribution, designing the modification-aware (MODA) page allocation scheme and analyzing how enhanced uniformity affects the block cleaning cost with various workloads. Real implementation experiments conducted on an embedded system show that the MODA scheme typically improves 20 to 30% in cleaning time compared to the traditional sequential allocation scheme that is used in YAFFS.

Flash memory has many merits such as light weight, shock resistance, and low power consumption, but also has limitations like the erase-before-write property. To overcome such limitations and to use it efficiently as storage media in mobile systems, Flash memory based storage systems require special address mapping software called the FTL (Flash-memory Translation Layer). Like cleaning in Log-structured file system (LFS), the FTL often performs a merge operation for block recycling and its efficiency affects the performance of the storage system. To reduce the block recycling costs in NAND Flash memory based storage, we introduce another block recycling scheme that we call migration. Our cost-models and experimental results show that cost-based selection of merge or migration for each block recycling can decrease block recycling costs and, therefore, improve performance of Flash memory based storage systems. Also, we derive the macroscopic optimal migration/merge sequence minimizing block recycling costs for each migration/merge combination period. Experimental results show that the performance of Flash memory based storage can be further improved by the macroscopic optimization than the simple cost-based selection.